A rotational joint is configured by using a motor and a reducer or using a hydraulic or pneumatic actuator, and in this case, when a hydraulic pressure is used, a cylinder-type actuator is mainly used.
In the case of a cylinder-type actuator, when a hydraulic actuator is particularly used, there is a merit of having a large output to volume ratio which is advantageous when a great amount of force or a dynamic motion is required.
In a cylinder-type actuator, as illustrated in FIG. 11A, even when a rod 1002 of a piston 1001 reciprocates, a sealing member 1004 is installed with respect to the rod 1002 to seal the rod 1002 so that a fluid f filling the inside of a cylinder 1003 does not leak.
Since a surface portion of the rod 1002 reciprocates while being in continuous frictional contact with the sealing member 1004, the surface of the rod 1002 should be smoothly processed to reduce friction, and thus, processing costs increase.
In addition, when a rotary joint is configured from a cylinder-type actuator in an excavator or the like, an operation range (driving angle) is restricted, and there is a limitation in that a complicated link mechanism is required to solve this.
To solve such a limitation, a tendon actuator of a Mckibben muscle-type which uses a cable or the like is disclosed in Japanese Patent Application Laid-open Publication No. 2012-125847.
In such a method, as illustrated in FIG. 11B, two first and second artificial muscles 2a and 2b are installed in parallel on a main body part 3.
First and second wires 5a and 5b are respectively connected to one end portions of respective artificial muscles 2a and 2b, and air supply parts 20a and 20b for supplying compressed air are respectively provided on the other end portions.
The first and second wires 5a and 5b are wound around a pulley 4 and a link part 6 is installed on the pulley 4.
The tendon actuator 2 is configured such that when compressed air is alternately supplied to each of the first and second artificial muscles 2a and 2b through air supply parts 20a and 20b, the first and second wires 5a and 5b are moved to thereby rotate the pulley 4, and thus, the link part 6 performs a joint motion while rotating.
However, in the case of such a tendon-type actuator, there is a limitation in that elastic deformation amounts of the first and second artificial muscles 2a and 2b are not so large that a sufficient operation range of the actuator cannot be ensured.
To solve such a limitation, an actuator of WC BRANHAM Co. in which the rod 1002 is substituted by a cable 1005 in the configuration of the conventional hydraulic or pneumatic actuator illustrated in FIG. 11A.
As illustrated in FIG. 11C, also in such a method, a problem of preventing leakage of fluid f between a cable 1005 and a sealing member 1004 may occur. To solve this, the cable 1005 is used such that the surface thereof is coated for increasing durability and preventing fluid leakage.
However, also in this case, the cable 1005 and the sealing member 1004 should be maintained to be in close contact with each other to prevent fluid leakage, and when the number of movements of the cable 1005 is increased due to a long operation time or the like, the sealing member 1004 is worn due to friction, and thus fluid leakage occurs.
In addition, there is a limitation in that the coating applied to the surface of the cable 1005 is damaged due to the contact and friction with a pulley 1006 while being wound and loosened around the pulley 1006, and the sealing between the cable 1005 and the sealing member 1004 is not ensured, thereby causing fluid leakage.